Pickling is a treatment of metallic surfaces before subsequent processing, such as extrusion, painting, galvanizing, or plating, comprising removing oxides and rust with a solution containing strong mineral acids with iron dissilving inhibitor additive, called pickling liquer. The two acids commonly used are hydrochloric acid and sulfuric acid. Spent pickle liquor is considered a hazardous waste by EPA. Once, spent pickle liquors were land disposed by steel manufacturers after lime neutralization, but nowadays it should rather be recycled or regenerated.
When manufacturing refined steel plates, pipes, etc., the steel plate is usually drawn through the acid base as a continuous strip. The hydrochloric or sulfuric acid in the basin gradually looses its pickling effect and reaches a maximum iron content, becoming a waste sludge. The basin is then emptied, and fresh acid is fed instead. There are several industrial methods of treating the galvanic waste sludge, which include three basic technologies. The first technology employs neutralizing the acid by Ca(OH)2, Na2CO3, NaOH, KOH, or NH4(OH), separating a solid precipitate, and recycling water. Such technology is described in U.S. Pat. Nos. 3,304,246, 3,544,309, and 3,800,024. In the last reference, the ions are selectively precipitated in two steps by adjustment of the pH value. The disadvantages of the neutralization methods include the loss of the acid, and the complexity of separating hydroxides and treating saline water, which also result in high costs.
The second technology includes heating in two stages of the galvanic waste sludge in order to evaporate water and to recycle acid (200-500° C.), followed by producing metal oxides during the calcination at high temperatures (850° C.). Such techniques are described in U.S. Pat. Nos. 4,197,139, 5,149,515, 5,244,551, and 6,451,280, relating to a process for regenerating spent acid liquor, comprising feeding into a reactor, having a first heating zone, a substantial portion of the liquid from the spent acid to produce acid vapors and metal salts without decomposing the acid. The metal salts are transferred to a second heating zone where the salts are roasted to form metal oxides. The acid vapors from the primary roasting furnace are then transferred to an absorption column to regenerate the acid. The first heating zone is operated at a temperature below the decomposition temperature of the acid and the metal salts. The second heating zone operates at a higher temperature to completely oxidize the metal salts. The disadvantages of the technology include very high energy consumption, production of dangerous, very corrosive gases, and low quality of produced metal oxides.
The third technology regenerates spent pickling acid by adding fresh strong acid to preconcentrated galvanic waste sludge, and manufactures iron salts by crystallization. The regenerated acid is reused in the pickling process, and the iron salt is sold as a by-product after washing. Such technology is described in U.S. Pat. Nos. 4,255,407, 4,382,916, WO/2001/049901 and WO/2009/075710. This technology is advantageous because it allows to regenerate spent pickling acid and to remove dissolved metal (for example iron) as a by-product (for example, iron sulfate heptahydrate). The technology is considered to be a progressive one, for example by Green Technology Group, which published it as an innovative closed-loop process (Published in the WEB www.oit.doe.gov in June 2000, Douglas Olsen Green Technology Group, Sharon, Conn.). Disadvantages may include high energy consumption for the acid evaporation or waste sludge concentration, difficulties with the acids separation and with product washing. It is therefore an object of the present invention to regenerate hydrochloric or sulfuric acid from galvanic waste sludge, and to recycle it to the repeated pickling process, without the drawbacks of the prior methods.
It is another object of this invention to regenerate hydrochloric or sulfuric acid from galvanic waste sludge or from used pickling liquor, while utilizing dissolved iron.
It is further an object of this invention to regenerate hydrochloric or sulfuric acid from galvanic waste sludge or from used pickling liquor, while utilizing dissolved iron, without neutralizing the acids or concentrating them by heat-evaporation.
It is still another object of this invention to recycle dissolved iron from the waste sludge or pickling used liquor by adding a material capable of precipitating said iron out of the solution and separating it.
It is a further object of this invention to recycle dissolved iron from the waste sludge or pickling liquor by precipitating and washing, followed by converting the washed material to a useful product
Other objects and advantages of present invention will appear as description proceeds.